1. Field of the Invention
The present invention relates to a film formation apparatus and a film formation method, which are used when a film formation material is purified by sublimation to form a film using the purified material in forming an EL element over a substrate.
2. Description of the Related Art
In recent years, there has been an active study of a light emitting device which has an EL element as a self light emitting element, and in particular, a light emitting device using an organic material as an EL material has attracted attention. This light emitting device is also called an organic EL display (OELD) or an organic light emitting diode (OLED).
Note that the EL element has a layer containing an organic compound in which electro luminescence is generated by applying electric field (hereinafter referred to as EL layer), an anode, and a cathode. The electro luminescence in the organic compound includes luminescence produced in returning from a singlet excitation state to a ground state (fluorescence) and luminescence produced in returning from a triplet excitation state to a ground state (phosphorescence). A light emitting device manufactured by a film formation apparatus and a film formation method according to the present invention can be applied to the case where either luminescence is used.
A light emitting device has a characteristic that a problem with respect to an angle of a field of view is not caused because it is a self light emitting type which is different from a liquid crystal display device. That is, it is more suitable than the liquid crystal display device when a display is used in the outdoors, and thus various ways of use are proposed.
The EL element has a structure in which an EL layer is interposed between a pair of electrodes. The EL layer generally has a laminate structure. Typically, there is a laminate structure called xe2x80x9chole transport layer/light emitting layer/electron transport layerxe2x80x9d, which is proposed by Tang, et al. of Eastman Kodak Company. Light emitting efficiency is very high in this structure, and thus, this structure is adapted to most of the light emitting devices which have been researched and developed at present.
In addition, a structure in which a hole injection layer, a hole transport layer, a light emitting layer, and an electron transport layer are laminated in this order on the anode is preferable. Also, a structure in which a hole injection layer, a hole transport layer, a light emitting layer, an electron transport layer, and an electron injection layer are laminated in this order on the anode is preferable. The light emitting layer may be doped with a fluorescent pigment or the like. These layers may be formed using a low molecular system material or a polymer system material.
Note that all layers provided between the cathode and the anode in this specification is generically called the EL layer. Thus, the hole injection layer, the hole transport layer, the light emitting layer, the electron transport layer, and the electron injection layer, which are described above, are all included in the EL layer.
Also, in this specification, a light emitting element formed of a cathode, an EL layer, and an anode is called the EL element. There are two types, that is, a type in which an EL layer is formed between two kinds of stripe-shaped electrodes provided to intersect each other (simple matrix type) and a type in which an EL layer is formed between a pixel electrode which is connected with a TFT and arranged in matrix and a counter electrode (active matrix type).
The most serious problem in putting the EL element to practical use is the insufficiency of the element life. Also, deterioration of the element is recognized in the form that a non light emitting region (dark spot) is expanded with light emission for a long period of time due to deterioration of the EL layer which eventually becomes a problem.
An EL material for forming the EL layer is deteriorated by an impurity such as oxygen, water, or the like. Also, when other impurity is included in the EL material, there arises a fear that the deterioration of the EL layer is adversely influenced.
Conventionally, when film formation is performed by an evaporation method, an evaporated material is used without being processed. However, it is considered that an impurity is mixed into the evaporated material at evaporation. That is, there is a possibility that oxygen, water, and other impurity are mixed as one reason for the deterioration of the EL element.
Also, when the evaporated material has been purified in advance, the purity can be increased. However, there is a possibility that an impurity is mixed during a period until the evaporation is completed.
The present invention has been made in view of the above problems, and an object of the present invention is therefore to provide a film formation apparatus with which an impurity included in an EL material at film formation is separated and removed, film formation is performed using an EL material with the increased purity, and thus an EL layer with high purity can be formed. Also, another object of the present invention is to provide a film formation method using the film formation apparatus of the present invention.
The present invention is characterized in that the EL material is purified by sublimation using a sublimation temperature of the pure EL material immediately before the film formation to remove plural impurities included in the EL material, and then a thin film is formed using the purified EL material (hereinafter referred to as high purity EL material) as an evaporation source.
In FIG. 1, it will be described that in the case where the EL material in which plural impurity substances are included is evaporated from a solid and a temperature is changed, plural substances can be separated from the EL material in accordance with different sublimation temperatures of respective substances. Note that an ordinate is given by a temperature and an abscissa is given by the total amount of precipitation. In this specification, a substance (impurity) which has a higher sublimation temperature than the high purity EL material is called a high temperature material, and a substance (impurity) which has a lower sublimation temperature is called a low temperature material. Also, the high purity EL material which is sublimated at a middle temperature located between a high temperature and a low temperature is called a middle temperature material. Note that, with examining a material precipitated at every temperature in advance by an analysis such as a mass analysis (GC-MS), a sublimation temperature of the pure EL material can be examined.
First, a temperature at which all substances (high purity EL material and impurity) included in the EL material are evaporated (hereinafter referred to as complete sublimation temperature) is set. Thus, there are all substances included in the EL material as gases in a complete sublimation region 100. Thereafter, when a temperature is gradually decreased, a high temperature material which has a high sublimation temperature is precipitated as a solid in a high temperature material precipitation region 101 shown in FIG. 1.
Further, when a temperature is decreased, a middle temperature material (high purity EL material) as a main product is precipitated in a middle temperature material precipitation region 102 shown in FIG. 1. Then, a temperature is again decreased, a low temperature material is precipitated in a low temperature material precipitation region 103.
That is, according to the present invention, there are provided a film formation apparatus and a film formation method for separating an impurity precipitated at a high temperature (high temperature material) or an impurity precipitated at a low temperature (low temperature material) from the high purity EL material (middle temperature material) based on a precipitation temperature difference and performing film formation using only the high purity EL material. Further, it is characterized in that not only a film formation material but also a film formation apparatus itself has various functions for obtaining high purity.
In FIGS. 2A and 2B, a method of performing film formation by evaporation after an EL material 200 is purified by sublimation will be described. FIGS. 2A and 2B show cross sectional views. First, as shown in FIG. 2A, separation of a low temperature material (impurity) is performed. A system has a temperature control mechanism. In the system, with respect to the EL material, a change in a state between a solid and a gas due to a temperature, that is, a phenomenon called sublimation is occurring. In a plurality of systems shown in FIGS. 2A and 2B, the change in the state is produced. The temperature control mechanism includes a heater as a concrete example. A system (1) 201 includes the EL material 200. Also, the system (1) 201 has a temperature control mechanism (a) 203 and thus a temperature can be controlled to a complete sublimation temperature for evaporating all the EL material. The EL material evaporated in such a system (1) 201 is called a gas EL material.
The gas EL material generated in the system (1) 201 is moved to a provided system (2) 204 as shown in FIG. 2A. The system (2) 204 has a temperature control mechanism (b) 205 which is capable of keeping the system (2) 204 at a middle temperature. Note that in the cross sectional view shown in FIG. 2A, a lower side of the system (2) 204 is coupled to the system (1) 201 so that all the gases produced in the system (1) 201 can be moved to the system (2) 204. Also, in an upper side portion, an opening 210 is provided so that the gases in the system (2) 204 can be passed therethrough.
Of the gas EL material moved to the system (2) 204, a substance sublimated at a middle temperature or higher is precipitated as a solid in a precipitation region (a) 211 inside the system (2) 204. In the specification of the present invention, the substance precipitated here is called a semi EL material. Note that a low temperature material sublimated at a middle temperature or lower is left as a gas and thus emitted from the opening 210 of the system (2) 204 to the outer portion thereof. The low temperature material emitted here is an impurity included in the EL material and thus recovered to the outside.
Next, the system (2) 204 in which the gases are removed is inversed by 180xc2x0 as shown in FIG. 2B. Then, the system (2) 204 is kept at a complete sublimation temperature by the temperature control mechanism (b) 205. Thus, the semi EL material precipitated inside the system (2) 204 is evaporated and moved to a system (3) 206.
The system (3) 206 has a temperature control mechanism (c) 207. Here, the system (3) 206 is kept at a high temperature by the temperature control mechanism (c) 207. At this time, of the semi EL material, a high temperature material (impurity) is precipitated in a region (b) 212 inside the system (3) 206. Thus, the high temperature material (impurity) can be separated and there can be only a middle temperature material (high purity EL material) as a gas inside the system (3) 206.
Also, as in the case of the system (2) 204 shown in FIG. 2A, the system (3) 206 has an opening 213 in its upper portion so that a gas can be passed therethrough. Thus, the high purity EL material (gas) can be emitted to the outside of the system (3) 206.
Then, a substrate 208 is provided in a direction along which the high purity EL material (gas) is emitted, and the high purity EL material obtained by purification with sublimation so far can be formed to be a film (evaporated).
Note that it is preferable that the substrate 208 is thermally treated immediately before film formation to remove an impurity such as oxygen or water on the substrate 208.
Further, as a material used for inside of a film formation chamber for performing purification of the EL material with sublimation and film formation, aluminum, stainless steel (SUS), or the like, which is electrolytic-polished to a mirror state, is used for the internal wall surface. This is because absorbability of the impurity such as oxygen or water can be reduced by decreasing its surface area. Thus, a degree of vacuum in the film formation chamber can be kept to be 10xe2x88x925 to 10xe2x88x926 Pa. Also, materials such as ceramics processed in order that the number of pores is minimized are used for the inner member. Note that these preferably have surface smoothness of which average roughness in the center line is 30 angstroms or less.
Also, when gases are introduced into respective processing chambers such as a film formation chamber and a transfer chamber, which are included in the film formation apparatus of the present invention, the impurity such as oxygen or water is removed by a gas purifying unit located immediately in front of the respective processing chambers, thereby introducing high purified gases thereinto.
Further, a magnetic levitation turbo molecular pump, a cryopump, or a dry pump is provided in each of all the processing chambers such as the film formation chamber and the transfer chamber, which are included in the film formation apparatus of the present invention. Thus, a degree of vacuum to be reached in the respective processing chambers can be set to be 10xe2x88x925 to 10xe2x88x926 Pa and back diffusion of the impurity from a pump side and an evacuation system can be controlled.
Note that, when an EL layer is formed by using the film formation apparatus of the present invention, surface processing of an anode or a cathode of an EL element which is formed on a substrate, is performed before forming the EL layer. As its concrete method, there is a method of performing thermal treatment with irradiating ultraviolet light in an oxygen atmosphere, a method of performing thermal treatment with conducting oxygen plasma processing or hydrogen plasma processing, or the like. Note that a heating temperature is preferably 100xc2x0 C. or lower. Also, it is effective that a mechanism for heating the film formation chamber at 100xc2x0 C. or lower is provided in the film formation chamber in order to remove the impurity in the film formation chamber before film formation.
Note that the purification with sublimation in the present invention can be applied to purification of not only the EL material but also of other material such as a metal material used for evacuation.